In recent years, due to serious issues such as the imminent exhaustion of fossil fuels, global warming caused by carbon dioxide emissions resulting from the use of fossil fuels, the accident of nuclear power plants, and radioactive contamination caused by nuclear waste, the development of next-generation clean energy sources has been of increasing importance. Among them, solar power generation systems use infinite resources and are semi-permanent, and thus are receiving as a next-generation energy source. Solar cells that are currently used for solar power generation include crystalline silicon solar cells, thin film silicon solar cells, CdTe solar cells, CIGS solar cells, dye-sensitized solar cells, organic solar cells, and concentrated solar cells. Currently, silicon solar cells make up the majority of the marker because of their guaranteed reliability. However, silicon solar cells include expensive silicon substrates and expensive silver and aluminum pastes. Thus, in order to lower the price of silicon solar cells to achieve grid parity, the expensive materials are required to be replaced with inexpensive materials.
A solar cell is a semiconductor device that converts solar light directly into electricity based on the photovoltaic effect in which electrons are produced when light is applied to a semiconductor diode that constitutes a p-n junction. In a method for fabricating a general silicon solar cell, a pentavalent element is thermally diffused into a p-type silicon substrate to form an n-type layer on the front surface of the silicon substrate, thereby forming a p-n junction in the silicon substrate. On the n-type layer, a silicon nitride film is deposited to form an antireflective layer. A front electrode on the antireflective layer on the silicon substrate is generally composed of a plurality of parallel finger lines having a narrow width (70-100 μm) and a plurality of bus bars perpendicular to the finger lines and having a large width (1.5-2 mm). The front electrode is formed by screen-printing silver paste at high temperature. The rear surface of the silicon substrate is completely covered with aluminum paste. The aluminum rear electrode may have poor electrical/mechanical contact with a metal ribbon that provides a connection between cells, due to the oxidation of its surface. For this reason, aluminum/silver paste is screen-printed on the rear electrode to form rear bus bars. The front and rear electrode pastes are calcined at a temperature of 800° C. At this time, the silver paste of the front electrode penetrates through the antireflective layer so as to be connected with the n-type layer.
A voltage that can be obtained in a unit silicon solar cell is generally 1V or lower, which is very lower than a practically useful level. For this reason, a silicon solar cell module that generates power using solar cells includes a plurality of solar cells connected in series and parallel so as to generate a desired voltage and current. As shown in FIG. 1, a general silicon solar cell module is fabricated by connecting the front electrode bus bars and rear electrode bus bars of silicon solar cells to one another by a metal ribbon to make a silicon solar cell array, encapsulating the array with an encapsulation resin including EVA (ethylene vinyl acetate) and PVB (poly vinyl butyral), and laminating a glass sheet on the front surface and a back sheet on the rear surface of the array.
Silver that is used in the front electrode and rear electrode bus bars of a silicon solar cell is an expensive rare metal. In recent years, the price of silver has recently increased rapidly. For this reason, it is required to reduce the use of silver or replace silver with other inexpensive materials. In order to form a front electrode using a paste of an inexpensive metal such as copper or nickel instead of silver, the metal paste that is used for the front electrode should satisfy the following requirements:
1) The resistivity of the metal paste is as low as that of a high temperature silver paste, so that the metal paste does not reduce the solar cell conversion efficiency.
2) The metal paste should be easily soldered to a metal ribbon that provides a connection between cells, so that it is electrically or mechanically easily connected with the metal ribbon.
3) Because a solar cell module should be used for 20 years or more in the open air, the metal paste should not undergo oxidative corrosion when it is used in this environment for a long period of time.
However, a paste of an inexpensive metal such as copper or nickel, which is currently used, produces a metal oxide film when it is calcined. This oxide film is a non-conductive, and thus has a problem in that it interferes with the electrical or mechanical connection between metal particles in the paste and between the front and rear electrode bus bars and the metal ribbon. Moreover, because the metal powder is surrounded by the polymer of the resin and strongly bonded with the resin, it is not easily soldered to the metal ribbon. Particularly, when the metal paste composed of copper powder is exposed to air or moisture for a long period of time, it can be oxidized (corroded) to increase the electrical resistance of the electrode to thereby reduce the light conversion efficiency of the solar cell module.
In the prior art, Korean Patent Laid-Open Publication No. 2010-75661 discloses a conductive paste for a solar cell device, which contains conductive particles, an organic binder, a solvent, glass frit, an organic compound containing an alkaline earth metal, and a low-melting-point metal or a low-melting-point metal-based compound. According to the disclosure of the above patent publication, an alkaline earth metal is used as a low-melting-point metal in order to prevent either micro-cracks from occurring or contact resistance from increasing when printing, drying and calcining a conductive paste on the surface of a semiconductor substrate. However, the above patent publication does not describe that finger lines and bus lines comprise different components.
In addition, Korean Patent Laid-Open Publication No. 2012-90249 discloses an interconnection perpendicular to a finger electrode. According to the disclosure of this patent publication, a bus bar electrode that intersects a finger electrode and is in a line with an interconnection may not be included while the bus bar electrode generally comprises an electrode paste composed of an expensive material such as silver (Ag). Also, it discloses that, when the bus bar electrode is omitted, the amount of electrode paste used to form the bus bar electrode can be reduced, and a process for forming the bus bar electrode can be omitted, thereby greatly reducing the production cost. In addition, it discloses that the interconnection can be connected to a plurality of finger electrodes by a conductive film or a conductive paste without using a bus bar electrode and that the conductive film comprises a plurality of conductive particles made of a metal (e.g., nickel (Ni)) in epoxy resin and also that the conductive particles may have a size of 3-10 μm. However, it does not recognize the effect of the difference in the paste component between the finger electrode and the bus bar on processes or costs.